Multiwavelength observations of cirrus clouds in the North Celestial Loop: a study of the OH emission

New 1665- and 1667-MHz OH observations were made with the Green Bank Telescope at an angular resolution of 7 arcmin of 108 locations over two diffuse gas clouds at high Galactic latitude. A linear correlation is observed between the IRAS 100 mu m infrared intensity, which traces all the gas, and the OH column density, indicating that the amount of OH increases with the visual extinction above a threshold value of A(V) approximate to 0.5. N(OH) increases monotonically with N(H i) in up to N(OH) approximate to 0.25-0.3 x 1014 cm-2. At greater values of N(OH) the H i column density saturates at N(H i) approximate to 5 x 1020 cm-2, suggesting that there is molecular gas not traced by H i. No correlation is found between the 12CO integrated intensity and the OH column density, indicating that OH might be a better tracer of H-2 than CO in those diffuse regions. An H i column density of at least approximate to 2.2-2.9 x 1020 cm-2 is needed for the formation of OH, but in regions where the cloud is less shielded from the interstellar radiation field the value rises to N(H i) = 3.3 x 1020 cm-2.